X-ray is widely used in various fields such as high-resolution medical imaging which includes digital angiography and X-ray tomography especially for bone density testing, portal imaging in radiotherapy and other non-destructive testing techniques using X-ray. As X-ray has negative effects on the health of the tested subjects, X-ray used for medical imaging is relatively weak in strength. How to quickly obtain high-resolution and low interference images with a relatively weak X-ray has become a highly desirable goal in the field.
As shown in FIG. 1, most X-ray detectors have two major parts: a convertor and an imaging apparatus. In the detectors currently available, the convertor is composed of a scintillator. During X-ray testing, the X-ray passes through the tested subject and hits the surface of the scintillator which absorbs the X-ray and generates the visible light. The imaging apparatus then takes images using the detected visible light. The media for imaging can be film, display or digital media.
The scintillator is usually made of rare-earth metal oxides such as Gd2O2S and CsI etc. After passing through the tested subject during the test, different beams of X-ray change their direction and hit the scintillator at different incident angles. The scattering and diffraction of the X-ray beams will result in lower photon flux. In addition, unparallel neighboring X-ray beams will cause interference between neighboring pixels of the finished image. All these factors combined will lower the MTF (Modulation Transfer Function) and DQE (Detective Quantum Efficiency) of the detector. MTF can characterize the image resolution and DQE can characterize the extent of interference between pixels.
At present, using new semiconductor materials to make the convertor is a way to improve the resolution and reduce the inter-pixel interference of the X-ray detector. One such new semiconductor material is CdSe which is able to directly convert X-ray to electronic signals, thus greatly improving the MTF and DQE of the convertor. However, this new semiconductor material is toxic and expensive, which limits its use.